Human dignity requires choice. One cannot be dignified if one cannot choose, in their own mind and for their own self, what changes they wish to make to the world. Liberty and dignity are inextricably linked. We owe no debt of allegiance to any government that imposes on our freedoms rather than staunchly protects them.
I've been trying to think of a clear and concise way to say this. That seems to be the simplest form.
Most people will agree with the essentially utilitarian view that the actions with the most good for the most people are right, or at least better, even if they do not agree with the overarching utilitarian theory. The problem, of course, is how to define good.
If health (which would include life, obviously enough) is the highest good, enslaving others to make them serve the common good becomes not only not objectionable, it becomes a mandate. Arguments from this standpoint are, therefore, extremely dangerous to make. Even if the value of the life of specific individuals can fluctuate, this is an unreasonable stance to take. If life is the highest good, all expenses must be taken to preserve and protect it. If this were true, all of the following would be consequentially true: All killing is evil, (which would include both abortion and capitol punishment) health care is the highest form of community service, the infirm and/or elderly should be cared for at any expense and without choice.
Some try to temper this by claiming "quality of life" is the highest good. That's another flawed yardstick, as it's a completely subjective scale. Within that framework, a valid defense for murder would be ending a poor quality of life. Similarly still, enslavement is only unethical as far as it imposes on the quality of the slave's life.
A better foundation is the dignity of man. Within this framework, a person can choose anything as their personal ethical foundation, as long as they respect the rights of others to do the same. Should a person think that the health of another is the best thing to spend their resources improving, they have that right. They even have the right to attempt to convince others to aid the action. If they wish to provide for their own life, they have that right, or if they put others first, they have that right as well.
The right they lack is to coerce or choose for another. Stealing is wrong because it subverts the owner's will more than because it removes some comfort from the owner's life, under this ethical system. Taxation should minimize the burden and loss of choice, for the same reasons. Slavery is clearly wrong under this institution because it subverts the will of the individual. (Of course, voluntary servitude is completely acceptable under this system, as it is a free choice.)